Preparation of esters of omega-amino acids



nited States PREPARATION OF ESTERS F w-AMINO ACIDS Robert H. Hasek, Kingsport, Tenn., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application July 1, 1954, Serial No. 440,843

6 Claims. (Cl. 260482) This invention relates to a new and improved method for preparing various w-amino acid esters. More particularly, the invention is concerned with the preparation of such acid esters by reductive amination of aldehydic acids and esters thereof.

It is known that primary amines can be prepared by hydrogenating the reaction product of an aldehyde and ammonia in the presence of a hydrogenation catalyst. Variations in the reaction conditions and use of primary amines in place of ammonia permit the production of secondary amines by this procedure. This reaction is described in some detail by W. S. Emerson, Organic Reactions, vol. IV, pages 175-255 (1948), John Wiley & Sons, Inc. It is also known that esters of organic carboa'ylic acids react with ammonia and alkylamines to give the corresponding amides. In view of this known reactivity of both aldehyde and ester groups with amines, it was surprising to find that certain compounds containin" both of these groups in the same molecule, for example, aliphatic aldehydic acid esters, under reductive amination reaction conditions, gave substantial yields of o-amino acid esters. Accordingly,'in my new process the ammonolysis of the ester group is completely suppressed or greatly minimized. The amino acid esters prepared by my method are valuable intermediates for the preparation of other organic compounds, and are especially useful for preparing w-amino acids and polyamides therefrom.

It is, accordingly, an object of my invention to provide a new and improved method for preparing esters of amino acids by reductive amination of esters of aldehydic acids. Another object is to provide a process wherein ammonolysis of the ester group is completely suppressed or greatly minimized. Another object is to provide a process for preparing unusually stable esters of amino acids. Other objects will become apparent from a consideration of the following description of my invention.

In accordance with the invention, I prepare the w-amino acid esters of my invention by reacting certain saturated aliphatic aldehydic acids, preferably alkyl esters thereof (alkyl aldehydates) with ammonia or with a primary alkylamine and with hydrogen, in the presence of a suitable hydrogenation catalyst. The reaction may be represented by the following equation:

wherein R represents an atom of hydrogen or an alkyl group containing from 1 to 8 carbon atoms, e. g. methyl and ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tort-butyl, octyl, etc., groups, R1 represents an alkyl group containing from 1 to 4 carbon atoms and R2 represents a divalent straight chain or branched chain alkylene group containing from 2 to 10 carbon atoms. The reaction with ammonia or with amines is usually carried out in a 2,777,873 Patented Jan, 15, .1957

single step, but where the intermediate aldimine is quite stable, the reaction can also be carried out in two steps, i. e. the aldimine is'first formed and then hydrogenated. The proportion of ammonia or the alkylamine can be used in amounts equivalent to the aldehydic ester, but advantageously from 2 to 20 moles or even more of ammo'nia or amine per mole of the aldehydic ester is. employed, the excess of ammonia or amine tending to suppress side reactions such as the formation of secondary amines, which presumably occurs by reductive amination of the aldehyde by the first formed primary amine. The best results are obtained under strictly anhydrous conditions. Advantageously, the reductive amination to w-amino acid esters can be carried out in an inert solvent medium such as a saturated monohydric aliphatic alcohol, e. g. methyl alco-' amination of the aldehydic ester with inactive amines containing 4 or more carbon atoms such as tert.-butylamine, tert.-octylamine, etc. Where ammonia or an unrestricted amine such as methylamine, ethylamine, butylamine, etc. as opposed to tert.-butylamine or tert.-octylamine is employed as the aminating agent, the aldehydic ester employed is preferably a secondary or tertiary ester,

'i. e. wherein R1 is a branched chain alkyl group such as isopropyl, see-butyl or tort-butyl groups. The presence of either factor-an aldehydic ester resistant to ammonolysis or an inactive amine-is sutficient for the conduct of the reductive amination; both factors may be used, but only one is necessary. In addition, it is possible to accomplish the reductive amination of an aldehydic ester when neither factor is present, but the yields of amino esters are lower. Benzene and other hydrocarbon solvents can also be employed. i

In general, the temperature of the reductive amination process of my invention must be adjusted for each individual case, to provide for a rapid reaction at the lowest practical temperature. Since it is desirable to avoid prolonged exposure of either-the reactants or the product to the increased reaction temperature, a continuous process is advantageous, or at least a gradual addition of the aldehydic ester to the reaction vessel charged with ammonia or amine, catalyst and solvent, and brought to reaction conditions. Reductive amination may be conducted in some cases at temperatures as low as C. and in some cases as high as 160 C., although this latter temperature is within the range where ammonolysis of esters is said to take place readily. Lower temperatures, provided a rapid reaction is still obtained, are distinctly more advantageous; higher temperatures tend to produce excessive ammonolysis. The preferred temperature range of my process is from to C.

The pressure of hydrogen required in the reductive amination according to my process is less critical than the temperature. Reductive amination may be accomplished at pressures of only one or two atmospheres, but it is preferred to use pressures from about 20 atmospheres to about 1000 atmospheres (from about 300 to 15,000 p. s. i.). This range of pressures has the distinct advantage of lowering the reaction temperature required for effective reductive amination. The use of extreme pressures, i. e. pressures above about 1000 atmospheres, can also be used but these conditions give rise to difii-, culties in design and operation of suitable pressure ves gamers sels. Such extreme pressures and requirement of ap; paratus-for their application are by no means necessary in the practice of my invention.

Suit b hydrog nation;catalystsizfor use in my processi are fineIy,dividedxmeta lic nickel; cobalt;v iron; platinum.

or: pa ladium in: such: forms as; Raney nickeli on cobalt, reduccdsmt r di obalt oxides, niekelisupported onakiesel guhr, colloidalplatinum;,arnorphousrplatinum from reduction of platinum oxide palladium support'edcncharcoal, etc... Particularly. activeiormsoofRaney nickel and cohaltcatalysts-can be prepared by: either Washing the active catalyst. (following its-preparation. by action of alkali; on nickel-aluminum orcobaltvaluminum; alloy) under an atmosphere of hydrogen; or byvaddition of; a trace; of chloroplatinic acid: tolan. ammoniacal suspension of, the catalyst... For furthendetailsyofihese:latter, very.

active catalysts, refcrencewangbe had-,toH. .Adkins and R. Billica, Organic. Syntheses volt, 29, page 24 1949) ,John Wiley. and, Sons,-;Inc;,,New.- York; and to D. R. Leveringet 211., J. Amer; Chem:..S oc., 72, page'l190 V 950).. Generally-,- the- -lower;reactionttemperatures are quir en; r la iv ly, high. concentrations ofv these catalysts, are usedg, Supported catalysts such asnickel on; k eselg-uhr; and; massive. catalysts such 1- as.v foraminate nicl el-. a.lumi;num are entirely; suitable: catalysts and are P rticularly. useful in continuous reaction; systems.

Suitable aldehydicesters.fonusein-myprocess-are those represented by the ,generalformula:

wherein R1 and. R2. have the, previously defined meanh1g8. Typical aldehydic esters includepropyl succinalde:

hydate, isopropyl succinaldehydate, sec..-buty1,succinaldehydate. tert.- butyl succinalde'hydate; the, corresponding.

propylisopropylsec.-butyl1, and tert..-butyl glutaraldehydates,. the corresponding propyh, isopropylsec.- butyl-, and .tcrt.-butyl .adip aldehydates, the corresponding PXOPYI',

isopropyl, sec,.-,butyl-, and, tert.,-bu,tyl i pimelaldehydates,

the corresponding propyl-,,- isopropyl-, sec.-butyl-, and tert.-butyl sebacaldehydates, I the. corresponding. esters of p-methylglutaraldehydic acid. 'y-;y-.dimethylglutaraldehydic.

acid, and-.thelike. Suitable, primary. alkylamines; con.-

taining from 1 to 8 carbon atomsiforgreactingywith the;

above defined. jesters according. to my invention; include methylaminq ethylamine, propylamine-,1 isopropylamine,

n-butylaminm, secrbutylamiue; tert. -butylamin e, corresponding. amylamines,.,hexylam ines, heptylamines, octylamines, etc. v

T he. invention is. illustrated, further; by. the; following examples..-

Examplexl A- l-liter rocking-typeautoclavewascharged: with 200 cc. of 'isopropylalcohol, 50g. of ammonia andS g: of Raney nickel. Hydrogen was addedto'apressure of-1,000 p. s.-i. and the autoclave was heated'to l4 O,'C. 75g. of isopropyl 'adipaldehydate was; pumped intothe autoclave; followed-by 30 cc: of isopropyl alcohol to-rinse the pumplines: After"one-hourat l40 C; and 1',800p. s. i.

pressure (more? hydrogen a wasadded to: maintainthis pressure during thereaction) e the autoclave was cooled and discharged. The reaction mixturewasfiltered, disjtilled' atatmospheriepressure to remove ammonia and the'bulk -of the'is'opropylgalcohol; The'distillation pressure was then reduced-to mm and-isopropyl =e-amino: caproate was received at 96 -104 (1/5 mm: pressure;

It hadia refractive index of i 1.437 Oat 207 6. The: yield of product was 53 g. equivalent to, about 70%; based on the weight of isopropyl Eadipaldehydate.

A solution of 15 g of-isopropyl-'e-am nocaproate (prepared as in abovecxample) in,25 cc. ofwater'was re- I fiuxed under a fractionating column,the'isopropyl alcoholwater azeotrope being removed"asit'formed during the The hydrolysis" was essentially-complete iir I hydrolysis; 12 hours; refluxing was continued for an additional 5 hours. Residual water was removed, by heatingjhev still residue in a steam bath overnight. 5 g. of the residue, which apparently was a polyamide of low molecular weight, was heated at 240 C. under an atmosphere of hydrogen for one hour and finally under vacuum for an hour. The sticking point oft'fibers drawn from the molten product was 195 -2009 C.

Example 2 50 r g". of'liquid :ammonia, 20D rec; of: isopropyl alcohol and 5 g. of. Raney nickel catalyst were charged to a' 1- liter stainless.steel,autoclave,.. hydrogen was added to 1,500 p. s. i; pressure ,anditlieautoclave was heated to 140 C. A solution of 93 g. of isopropyl pimelaldehydate in 100 cc. of isopropyl alcohohwas pumped into the autoclave over a period of 21 minutes, and the autoclave was cooled immediately afterwards. The reaction mixture was; .filterede and? distilledtoflgive 4256 g; of. crude amino ester; which was: redistilled-r. to/ give isopropyl-teammaenanthate B. B. 10.4 'C;/5::mm-; pressure; specific gravity at; 20 (3; .ofxapproximately; 0921 and azrefractive index I 12 of approximately 1.439.

1O .g.'.of isopropyl--aminoeuanthate; prepared'as above described and- 15. cc; ofewaterwerexrefluxed for 16 :hours and thesisopropyl alcohol was removed. as: it formed by azeotropic distillation ofsisopropylalcohol-water mixture.

7 g. of:the.-.crude vproducti-obtained-.by evaporation of the still residuerwas-heated:in aseaIed gIass tube at 240* C. for;2-hours,; thenunder hydrogen-for O.5.hour and: in vacuum for-0.5:hour. The. polymer. which had formed had good colorand fibersdrawnfrom the melt exhibited the typical propertyrof cold drawing;

lsopropyl-g'eaminoenanthate prepared: as above described :was.-hydrolyzed1by.:being refluxed overnight in an equal volume 0fjdlhlte (:l': '1') hydrochloricacid, and the solution was ...evaporatedtodryness. The: crude hydrochloride $31115 (22 gz) awas dissolved: im 200 ce; of water and passed through acolumn of' ion exchange resin. 8 g.

of't-aminoenanthic acidiwas recovered; Recrystallization from: aqueous alcohol= gave colorless: crystals; M. P.

195--196 "C.. This material was polymerized bybeing:

was heated .at:l57 CZ" ZHd-EEIISOlUfiOHTOf 86 g. (0.5 'mol.)

of isopropyl adipaldehydatezin cc. of isopropyl al cohol'. was llJjQCtBdjLOVBY' aperiod of r 45 minutes. Hydrogen-was added periodically to'imaintain the reactionpressure;

was. cooled; vented :and. discharged..- The. reaction .prodnot, worked up in the mannerzoflExampleul, yielded 47 g. of isopropyl e-aminocaproate, boiling at 92-l02 C./5 mm. pressure, equivalent to a yield ofrapproximately 55% based on the weight of isopropyl adipaldehydate:

Example 4 Amixture of 100 g of ethyl adipaldehydate, 150 cc. of ethanol and-20g: ofjRaney' nickelwas;placed-' in a l-liter rockingtype'autoclave, 100' g; of anhydrous ammonia wasadde'd, and hydrogen-was pressed'in to a total pressure of 1500p. s; i: The autoclave was'heated'to C. for 25 minutes, and then .cooled, vented and discharged. The reaction product wasfil'tered; the excess'of ammoniaand solvent'were removed .by-heating; the filtrate ona'steam-b'atlt; and-the residue was'dis'tilled under reduced pressure. A yield of 37 g. of crude ethyl Aiteri thez additionn-of .the: ester, the pressure. was raised .to-ZSOO-m. s.'.i;, .byipaddition :of hydrogen, and heating-.wasaccntinuedrfor 30iminutest. Theautoclave.

e-aminocaproate, boiling at 8085 C./2 mm. pressure, was obtained which was equivalent to about 37%, based on the weight of ethyl adipaldehydate.

Example 5 A l-liter stirred autoclave was charged with 200 cc. of isopropyl alcohol, 5 g. of Raney cobalt catalyst, 47 g. of anhydrous ammonia and hydrogen to 1500 p. s. i. pressure. The autoclave was heated to 140 C. and a solution of 100 g. of sec-butyl pimelaldehydate in 100 cc. of isopropyl alcohol was pumped in over a period of 35 minutes. The autoclave was cooled immediately, vented and discharged. The reaction mixture was filtered to remove the catalyst and distilled at atmospheric pressure to a liquid temperature of 100 C. The residue was distilled under reduced pressure to give 53.5 g. of sec.- butyl f-aminoenanthate, boiling at 96-l06 C./5 mm. pressure, equivalent to a yield of approximately 53.5% based on the weight of sec-butyl pimelaldehydate.

Example 6 A l-liter stirred autoclave was charged with 200 cc. of isopropyl alcohol, 5 g. of Raney cobalt catalyst, 53 g. of anhydrous ammonia and hydrogen to 1500 p. s. i. pressure. The autoclave was heated to 150 C. and a solution of 44 'g. of tert.-butyl pimelaldehydate in 50 cc. of isopropyl alcohol was pumped in over a period of 7 minutes. The autoclave was cooled immediately, vented and discharged. The reaction mixture was worked up as in Example 4 to give 16 g. of tert.-butyl -aminoenanthate, boiling point 97-104 C./2.5-3.0 mm. pressure, which was equivalent to about 37%, based on the weight of tert-butyl pimelaldehydate.

Example 7 158 g. of ethyl adipaldehydate was added gradually to a solution of 129 g. of tert.-octylamine in 200 cc. of thiophene-free benzene. The milky solution was refluxed under a Dean-Starke trap until no more water was evolved. The benzene solution of the aldimine which formed was then placed in an autoclave with 5 g. of Raney nickel and treated with hydrogen at 1500 p. s. i. pressure and 118-125 C. for 1.5 hours. The catalyst was removed by filtration and the residual liquid was distilled through a 40 cm. Vigreux column under reduced pressure. A yield of 155 g. of ethyl e-tert.-octylaminocaproate, boiling at 139146 C./5 mm. pressure n 1.4509 at 20 C., was obtained, which was equivalent to approximately 57%, based on the weight of ethyl adipaldehydate.

By proceeding in similar manner as set forth in the preceding examples, other w-amino acid esters and N-alkyl substituted w-amino acid esters can be prepared from the various mentioned reactants. It has been indicated that addition of the aldehydic ester to the ammonia or amine solution in the reaction vessel is a desirable technique. In cases where the intermediate aldimine is relatively unstable, this technique is preferred; the formation and hydrogenation of the aldimine taking place in rapid succession in the reaction vessel. However, in certain cases, as in Example 7, a stable aldimine may be preformed and even isolated, the hydrogenation then being done as a distinctly separate step.

What I claim is:

1. A method for the preparation of w-amino acid esters which comprises heating an anhydrous aldehydic ester having the general formula:

with an anhydrous nitrogen compound having the general formula:

RNH:

wherein R represents a member selected from the group consisting of an atom of hydrogen and an alkyl group containing from 1 to 8 carbon atoms, R1 represents an alkyl group containing from 1 to 4 carbon atoms and R2 represents a divalent alkylene group containing from 2 to 10 carbon atoms in an anhydrous solvent medium selected from the group consisting of a saturated monohydric aliphatic alcohol containing from 1 to 4 carbon atoms and benzene, at from to C. and a pressure of from 1 to 1000 atmospheres, in the presence of hydrogen and a hydrogenation catalyst, in the proportions of from 1 to 20 moles of the said nitrogen compound to each mole of the said aldehydic ester, and recovering the formed w-amino acid ester from the reaction mixture.

2. A method for the preparation of isopropyl e-aminocaproate which comprises heating anhydrous isopropyl adipaldehydate with anhydrous ammonia, in anhydrous isopropylalcohol, at from 115 to 160 C., and at a pressure of from 20 to 1000 atmospheres, in the presence of hydrogen and a hydrogenation catalyst, in the proportions of from 1 to 20 moles of ammonia to each mole of the isopropyl aldehydate, and recovering the formed isopropyl e-aminocaproate from the reaction mixture.

3. A method for the preparation of ethyl e-aminocaproate which comprises heating anhydrous ethyl adipaldehydate with anhydrous ammonia in anhydrous ethyl alcohol, at from 115 to 160 C. and at a pressure of from 20 to 1000 atmospheres, in the presence of hydrogen and a hydrogenation catalyst, in the proportions of from 1 to 20 moles of ammonia to each mole of the ethyl adipaldehydate, and recovering the formed ethyl e-amino-caproate from the reaction mixture.

4. A method for the preparation of see-butyl {-aminoenanthate which comprises heating anhydrous sec.-butyl pimelaldehydate with anhydrous ammonia, in anhydrous isopropyl alcohol, at from 115 to 160 C. and at a pressure of from 20 to 1000 atmospheres, in the presence of hydrogen and a hydrogenation catalyst, in the proportions of from 1 to 20 moles of ammonia to each mole of the sec.-buty1 pimelaldehydate, and separating the formed sec-butyl {-aminoenanthate from the reaction mixture.

5. A method for the preparation of tert.-butyl {-aminoenanthate which comprises heating anhydrous tert.-butyl pimelaldehydate with anhydrous ammonia, in anhydrous isopropyl alcohol, at from 115 to 160 C. and at a pressure of from 20 to 1000 atmospheres, in the presence of hydrogen and a hydrogenation catalyst, in the proportions of from 1 to 20 moles of ammonia to each mole of the tert.-butyl pimelaldehydate, and separating the formed tert.-butyl {-aminoenanthate from the reaction mixture.

6. A method for the preparation of ethyl e-terL-octylaminocaproate which comprises heating anhydrous ethyl adipaldehydate with anhydrous tert.-octylamine, in anhydrous benzene, at from 115 to 160 C. and at a pressure of from 20 to 1000 atmospheres, in the presence of hydrogen and a hydrogenation catalyst, in the proportions of from 1 to 20 moles of the tert.-octylamine to each mole of the ethyl adipaldehydate, and separating the formed ethyl e-terL-octylaminocaproate from the reaction mixture.

References Cited in the file of this patent UNITED STATES PATENTS Moe et a1. Apr. 3, 1951 Whetstone et a1. Apr. 21, 1953 

1. A METHOD FOR THE PREPARATION OF W-AMINO ACID ESTERS WHICH COMPRISES HEATING AN ANHYDROUS ALDEHYDIC ESTER HAVING THE GENERAL FORMULA: 